Here's an excerpt from the Yahoo! news article:

"The White House's site uses what's known as a Web bug — a tiny graphic image that's virtually invisible — to anonymously keep track of the number and time of visits. The bug is sent by a server maintained by an outside contractor, WebTrends Inc., and lets the traffic-analysis company know that another person has visited a specific page on the site."

Here's the link to the news:

http://news.yahoo.com/s/ap/20051231/ap_on_hi_te/white_house_bug;_ylt=Aty...